Field
The present disclosure relates to the moderation of neutrons in a nuclear reactor.
Description of Related Art
Conventionally, fast neutrons are produced by fission reactions in a nuclear reactor. A fast neutron is a free neutron with a kinetic energy level of about 1 MeV or more. Moderation is the process of reducing the initial high kinetic energy of the fast neutrons so as to convert the fast neutrons to lower-energy thermal neutrons. Thermal neutrons help sustain the chain of fission reactions in the core.
In a conventional nuclear power plant, water has been used as a neutron moderator to slow down (thermalize) the fast neutrons, with the water flowing upwardly through the fuel bundles. However, conventional fuel bundles operate with axially varying amounts of moderation in their active fuel region due to the eventual boiling of the water along the upper portions of the fuel length. In particular, the upper boiling regions have a reduced moderating capability relative to the lower non-boiling regions of the fuel bundles. As a result, there are more fission reactions and, thus, more power generated in the lower non-boiling regions than in the upper boiling regions of the fuel bundles, thereby creating a non-uniform axial power shape.
A non-uniform axial power shape can limit the overall reactor power generation based on the locations of the power peaks and the design limitations of those locations. Additionally, the non-uniform axial power shape can cause the fuel in the bottom of the fuel bundles to be consumed at a faster rate than the fuel in the top of the fuel bundles. Consequently, when the lower sections of the fuel bundles burn out, a large portion of the fuel in the upper sections of the fuel bundles may still remain unburned, resulting in poor fuel utilization. Furthermore, poor neutron moderation in the upper sections of the fuel bundles results in increased fast neutron irradiation. Accordingly, various structures within a nuclear reactor may be degraded over time by the fast neutron irradiation, thereby shortening the life of those components and requiring mitigating action to continue operation of the nuclear power plant.